Electronic computing and communications systems continue to include greater numbers of features and to increase in complexity. At the same time, electronic computing and communications systems decrease in physical size and cost per function. Rapid advances in semiconductor technology such as four-layer deep-sub-micron complimentary metal-oxide semiconductor (CMOS) technology, have enabled true "system-on-a-chip" designs. These complex designs may incorporate, for example, one or more processor cores, a digital signal processing (DSP) core, several communications interfaces, and graphics support in application-specific logic. In some systems, one or several of these extremely complex chips must communicate with each other and with other system components. Significant new challenges arise in the integration, verification and testing of such systems because efficient communication must take place between sub-systems on a single complex chip as well as between chips on a system board. One benefit to having an efficient and flexible method for communication between sub-systems and chips is that system components can be reused in other systems with a minimum of redesign.
One challenge in the integration, verification and testing of modern electronic systems stems from the fact that modern electronic systems in many application areas have functionality, cost and form-factor requirements that mandate the sharing of resources, such as memory, among multiple functional blocks, where functional blocks can be any entity that interfaces to a communication system. In such systems, the functional blocks typically possess different performance characteristics and requirements, and the communications system and shared resources must simultaneously satisfy the total requirements. Key requirements of typical functional blocks are bandwidth and latency constraints that can vary over several orders of magnitude between functional blocks. In order to simultaneously satisfy constraints that vary so widely, communications systems must provide high degrees of predictability.
Traditional approaches to the design of communications systems for modern, complex computer systems have various strengths and weaknesses. An essential aspect of such approaches is the communications interface that various sub-systems present to one another. One approach is to define customized point-to-point interfaces between a sub-system and each peer with which it must communicate. This customized approach offers protocol simplicity, guaranteed performance, and isolation from dependencies on unrelated sub-systems. Customized interfaces, however, are by their nature inflexible. The addition of a new sub-system with a different interface requires design rework.
A second approach is to define a system using standardized interfaces. Many standardized interfaces are based on pre-established computer bus protocols. The use of computer buses allows flexibility in system design, since as many different functional blocks may be connected together as required by the system, as long as the bus has sufficient performance. It is also necessary to allocate access to the bus among various sub-systems. In the case of computer buses, resource allocation is typically referred to as arbitration.
One disadvantage of computer buses is that each sub-system or component connected to the bus is constrained to use the protocol of the bus. In some cases, this limits the performance of the sub-system. For example, a sub-system may be capable of handling multiple transaction streams simultaneously, but the bus protocol is not capable of fully supporting concurrent operations. In the case of a sub-system handling multiple transaction streams where each transaction stream has ordering constraints, it is necessary for the sub-system to identify each increment of data received or transmitted with a certain part of a certain data stream to distinguish between streams and to preserve order within a stream. This includes identifying a sub-system that is a source of a data transmission. Conventionally, such identification is limited to a non-configurable hardware identifier that is generated by a particular sub-system or component.
Current bus systems provide limited capability to preserve order in one transaction stream by supporting "split transactions" in which data from one transaction may be interleaved with data from another transaction in the same stream. In such a bus, data is tagged as belonging to one stream of data, so that it can be identified even if it arrives out of order. This requires the receiving sub-system to decode an arriving address to extract the identification information.
Current bus systems do not support true concurrency of operations for a sub-system that can process multiple streams of transactions over a single interconnect, such as a memory controller that handles access to a single dynamic random access memory (DRAM) for several clients of the DRAM. A DRAM controller may require information related to a source of an access request, a priority of an access request, ordering requirements, etc. Current communication systems do not provide for such information to be transmitted with data without placing an additional burden on the sub-system to adapt to the existing protocol.
In order for many sub-systems to operate in conventional systems using all of their capabilities, additional knowledge must be designed into the sub-systems to provide communication over existing communication systems. This makes sub-systems more expensive and less flexible in the event the sub-system is later required to communicate with new sub-systems or components. Existing communication approaches thus do not meet the requirements of today's large, complex electronics systems. Therefore, it is desirable for a communications system and mechanism to allow sub-systems of a large, complex electronics system to inter-operate efficiently regardless of their varying performance characteristics and requirements.